Short and long term response to fire of the giant cactus Trichocereus terschekii: survival, growth and reproductive output

Project Leaders:

Diego E. Gurvich & Pedro Jaureguiberry, Universidad Nacional de Córdoba, Argentina

Cacti are highly relevant across dry ecosystems of America, but studies on the effects of fires on these important plants are scarce. Our study focuses on the giant cactus Trichocereus terschekii from Northwestern Argentina, locally known as cardon. Despite its ecological and social relevance, and the important role of fire as an environmental factor in its distribution range, the effects of fire on this species have not been studied. The prevalence of T. terschekii makes it a key species, determining ecosystem functioning. Previous studies on similar species (e.g. the saguaro) showed high post-fire mortality and sporadic seedling recruitment, linked to wet years. Recruitment of T. terschekii occurs only from seed (asexual reproduction has not been observed). Therefore, if the mortality is high, it would be important to assess the possible recovery of the population from seeds.

We aim to measure post-fire recovery at two nearby sites that burned 18 years apart (in 2003 and 2021), allowing us to assess the short-term effect of fire, and in particular what factors determine survival, as well as the long-term effects. The oldest fire allows us to measure the long-term survival and the growth of individuals, as well as possible recruitment from seed. To do this we will measure 150 individuals in each fire, as well as unburned control sites. We propose to measure several variables that could be related to both short-term survival and long-term recovery, including micro-environmental characteristics (e.g. cover of rocks, bare soil, vegetation, slope angle and orientation), that could relate to fire severity and influence post-fire dynamics.

We expect to have low post-fire survival and also limited recruitment. We believe that our study will be a valuable contribution to the understanding of the post-fire dynamics of cacti and to the design of proper management and conservation strategies in the region. It could also be used to prevent damage in the future and/or to restore burned sites.

Figure 1a

Fig. 1. Many individuals of T. terschekii fail to survive fire (1a and 1b), while a few survive and resprout (1c and 1d).

Figure 1b
Figure 1c
Figure 1d
Figure 2a
Figure 2b

Fig. 2. Fire intensity is an important factor in determining the post-fire performance of cacti: some are killed by fire (2a) and others survive (2b).

Figure 3

Fig. 3. Micro-environmental conditions nearby cacti, such as rock cover, slope and vegetation cover and type, can influence fire dynamics and therefore postfire response of cacti.

Figure 4

Fig. 4. The remains of standing shrubs can be used to estimate the severity of the fire.

Preliminary Conclusions as of the start of 2024:

• We observed low survival of T. terschekii exposed to low- and moderate-intensity fires. See Figure 5.
• Survival depends on plant size and some microenvironmental characteristics (likely biomass, which is related to severity at the individual plot level).
• Post-fire growth via resprouting was not related to plant size, but rather to the level of damage to the bark, indicating the relevance of remaining photosynthetic tissue after the fire.
• Severity estimated by remote sensing tools, such as the dNBR index, could be useful for improving survival estimates at the landscape level.

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